1. Field of the Invention
The present invention relates to a sheet conveyance apparatus configured to convey a sheet and to an image forming apparatus equipped with the sheet conveyance apparatus.
2. Description of the Related Art
Conventionally, an image forming apparatus is provided with a skew correction mechanism for correcting a skew of a sheet being conveyed, in order to align a sheet with a position of an image formed on the sheet, and a width-direction correction mechanism for correcting a position in a direction which perpendicularly crosses a conveyance direction of sheet (hereinafter, referred to as a width direction of sheet).
The skew correction mechanism is a mechanism, when the sheet being conveyed becomes skewed, for correcting the sheet orientation. The width-direction correction mechanism is a mechanism for correcting a deviation from a conveyance reference position in the width direction of the sheet which occurs in the middle of conveyance of the sheet or at the time of the skew correction by the skew correction mechanism.
Japanese Patent Application Laid-Open No. 59-4552 discusses a configuration, as a skew correction mechanism, for causing a leading edge of a sheet conveyed by a roller pair (hereinafter, referred to as a conveyance roller pair) provided upstream of a registration roller pair to contact the registration roller pair to allow the sheet to form a loop thereon, thereby correcting a skew of the leading edge portion of the sheet. Further, Japanese Patent Application Laid-Open No. 59-4552 discusses a configuration, as the width-direction correction mechanism, for correcting a position in the width direction of the sheet by moving the registration roller pair in the width direction of the sheet, while nipping the sheet by the conveyance roller pair and the registration roller pair.
However, as described above, when the image forming apparatus corrects the skew by forming the loop on the sheet, or, when the image forming apparatus moves the registration roller pair in the width direction, the sheet being nipped by the conveyance roller pair is twisted to cause the skew or wrinkles on the sheet in some cases.
Thus, Japanese Patent No. 3191834 discusses a sheet conveyance apparatus configured to allow the conveyance roller pair to be movable in the width direction of the sheet while nipping the sheet, in order to prevent a twist of the sheet. In the sheet conveyance apparatus, by arranging compression springs at both end portions in the width direction of the conveyance roller pair, the conveyance roller pair is movable in the width direction from a home position while following the sheet, without using a driving source. Then, the conveyance roller pair, which has moved in the width direction while following the preceding sheet, needs to return to a position before the movement (the home position), before a trailing edge of the preceding sheet passes through a rear end of the conveyance roller pair and a leading edge of the subsequent sheet reaches the conveyance roller pair. In the configuration discussed in Japanese Patent No. 3191834, the conveyance roller pair, which has moved while following the sheet, is returned from the moved position to the home position, by an elastic force of compression springs provided at both end portions in the width direction of the conveyance roller pair.
In the configuration discussed in the above-described Japanese Patent No. 3191834, when the conveyance roller pair is situated at the home position, elastic forces from both directions of the compression springs provided at both end portions in the width direction are balanced. However, in a case where the conveyance roller pair is situated at a deviated position from the home position, a difference between the elastic forces of the compression springs provided both end portions is equivalent to a force for returning the conveyance roller pair from the deviated position to the home position. As a result, the returning force becomes smaller. This is because a force for returning the conveyance roller pair to the home position by the compression spring at one side is weakened by a force in a reverse direction by the compression spring at the other side. As a result, when the conveyance roller pair, which has moved while following the sheet, returns from the moved position to the home position after the trailing edge of the sheet has passed through the conveyance roller pair, the conveyance roller pair may stop, in some cases, at a position deviated from the home position. In particular, since a force applied by the compression springs becomes weak in the vicinity of the home position, and a force for returning the conveyance roller pair to the home position becomes weak, the conveyance roller pair tends to stop in the middle of returning to the home position.
In this way, if the conveyance roller pair stops at the deviated position without returning to the home position, the sheet cannot be conveyed properly, that is, the sheet is skewed, and a position in the width direction of the sheet is deviated.
In this case, the conveyance roller pair may be configured to increase a force for returning from a position, at which the conveyance roller pair is deviated, to the home position, by strengthening an elastic force of the compression springs by increasing a spring constant of the compression springs, for example, provided at both end portions in the width direction of the conveyance roller pair. However, when the elastic force of the compression springs provided at both end portions in the width direction of the conveyance roller pair is strengthened, a force for causing the conveyance roller pair to move while following the sheet becomes large. As a result, that causes a twist, or a skew, or wrinkles of the sheet.